Studies on molecules encoded at the major histocompatibility complex of the rabbit (RLA) have used monoclonal antibodies (MCAb) prepared against fractions of a rabbit T cell. A 42,000 dalton molecule free of beta-2-microglobulin (B2-m) was shown by NH2-terminal sequence analysis and 2-D gel electrophoresis to be identical to RLA molecules isolated by affinity for anti-B2-m. In spite of apparent identity, the 42,000 dalton molecule associated with B2-m and that recognized by the MCAb comprise serologically distinct entities. Immunofluorescent studies indicate that the chains reactive with the MCAb are located in the cell cytoplasm. Studies on human histocompatibility antigens have begun with development of radioimmune assays for quantitation of HLA-A, -B, -C and -DR antigens. These assays have been used to determine optimum procedures for isolation of HLA molecules from lymphoid cell lines. The HLA-A3 antigen is being studied in detail to determine if multiple structural forms of this molecule correspond to differences observed in recognition of A3 cells by cytolytic T cells.